Breast cancer is a type of cancer that forms in tissues of the breast, usually the ducts and lobules. It occurs in both men and women, although male breast cancer is rare. It is estimated that in the United States approximately 230,000 new cases of breast cancer will arise in the year 2011, and about 40,000 deaths will occur that result from this form of cancer. See the website of the National Cancer Institute (NCI) at www.cancer.gov.
The ErbB2 (Her2/Neu) oncogene is overexpressed in 20-30% of human breast cancers and this overexpression is associated with poor prognosis and poor response to chemotherapy. ErbB2 is a 185-kDa type I tyrosine kinase transmembrane receptor that is a member of the epidermal growth factor receptor (EGFR) family. This family includes EGFR, ErbB2, Her3 and Her4. There is no known ligand for ErbB2, but this receptor has been shown to be the preferential heterodimerization partner for other ErbB family members that bind growth factors in the EGF, transforming growth factor-β, and heregulin families. The ErbB2 pathway promotes cell growth and division when it is functioning normally. While the precise mechanism of ErbB2 pathway activation in ErbB2-overexpressing cells is not entirely understood, overexpression likely leads to increased cell growth. See Chan et al., 2005, Breast Cancer Res. Treat., 91:187-201.
Trastuzumab (marketed under the name Herceptin® by Genentech) is a recombinant humanized monoclonal antibody that binds to the extracellular segment of the ErbB2 receptor. Trastuzumab is used as a single agent or in combination with chemotherapy and other targeted therapies to treat patients with breast cancer overexpressing ErbB2. Trastuzumab shows considerable clinical efficacy and has been shown to extend the overall survival of certain patients with ErbB2-overexpressing breast cancer. See Chan et al., 2005, Breast Cancer Res. Treat., 91:187-201.
Despite trastuzumab's general clinical efficacy of about 50% responsiveness, many patients do not respond to trastuzumab treatment at all (de novo nonresponsiveness), or acquire nonresponsiveness to trastuzumab treatment during the course of treatment. Postulated mechanisms of trastuzumab nonresponsiveness include: activation of the phosphoinositide 3-kinase (PI3K) pathway due to, for example, mutations in the PIK3CA gene; lack or inactivity of the tumor suppressor PTEN (phosphatase and tensin homolog); accumulation of truncated ErbB2 receptors (p95HER2) that cannot be inactivated by trastuzumab because they lack the extracellular domain to which trastuzumab usually binds; and overexpression of other RTKs that compensates for trastuzumab-induced ErbB2 inhibition. Examples of such RTKs include members of the epidermal growth factor receptor (EGFR) family, the insulin-like growth factor-1 receptor (IGF-1R) and the hepatocyte growth factor receptor (HGFR). See Zhang et al., 2011, Nat. Med., 17(4):461-468; see also Chan et al., 2005, Breast Cancer Res. Treat., 91:187-201.
Recently, it was demonstrated that the SRC kinase is a common node downstream of multiple pathways that result in tumors that are de novo nonresponsive or that have acquired nonresponsiveness to trastuzumab. See Zhang et al., 2011, Nat. Med., 17(4):461-468. The non-receptor tyrosine kinase SRC is a cytoplasmic protein that consists of three domains, an N-terminal SH3 domain, a central SH2 domain and a tyrosine kinase domain. SRC facilitates intracellular signal transduction by interacting with multiple RTKs through its SH2 domain and by phosphorylating and thus activating downstream targets. Examples of pathways and proteins activated by the SRC kinase include the AKT and the MAPK (mitogen-activated protein kinases) pathways, FAK (focal adhesion kinase), STAT3 (signal transducer and activator of transcription-3) and c-MYC. These signaling pathways and proteins have diverse roles in regulating tumor cell survival and metastasis. See Zhang et al., 2011, Nat. Med., 17(4):461-468.
It was found that SRC is activated (i.e., phosphorylated) in a model of acquired trastuzumab nonresponsiveness, wherein cultured cells overexpress EGFR or IGF-1R. Moreover, SRC is activated in PTEN-deficient cells in a model of de novo trastuzumab nonresponsiveness and in vitro GST pull-down assays demonstrated that SRC is a direct target of PTEN's phosphatase activity. On the other hand, SRC is inactivated (i.e., dephosphorylated) when, for example, the expression of EGFR is reduced, or when originally PTEN-deficient cells are reconstituted with wildtype PTEN. Furthermore, it was found that certain cells stably expressing a constitutively active SRC mutant are highly resistant to trastuzumab-mediated growth inhibition in vitro and in vivo, suggesting that SRC activation is sufficient to confer trastuzumab nonresponsiveness. The same study showed that SRC activity in human cancer specimens positively correlates with a lower clinical rate of response to trastuzumab treatment and that inhibition of SRC by saracatinib increases responsiveness of tumors to trastuzumab. See Zhang et al., 2011, Nat. Med., 17(4):461-468.